VASCULAR SYSTEM OF THE FERTILE FLORET OF PANICUM (GRAMINEAE: PANICOIDEAE: PANICEAE)

Serial transections of the rachilla of Panicum were used to reconstruct the vascular system of its fertile floret at anthesis. A developmental bias to the description of the floret vascular system furnishes new aspects of comparison within the subfamily Panicoideae and, outside of it, with the floret vascular systems of other subfamilies. The two collateral bundles descending the styles merge below the locule of the ovary of P. dichotomiflorum. The merging stylar bundles are joined by the descending placental bundle. The three blend into an amphivasal bundle that continues descending the rachilla as the pistil prong. It descends in the posterior of the rachilla until the level of entry and merger of the three stamen traces at the anterior of the rachilla. Horizontal unions of sieve and tracheary elements connect these erstwhile independent components. Intermediary cells specialized as transfer cells are abundant in the rachilla at this level. In the lower regions of this major interconnection, a minor trace from each of the two lodicules joins the entering lateral stamen traces. The anterior stamen trace is the major component that continues descending the anterior of the rachilla beyond the horizontal connection of stamen traces and pistil prong. In the rachilla at the lowest levels of the fertile floret, the median trace of the lemma joins, as a minor vein, the descending anterior stamen trace.